1. Field of the Invention
The present invention relates to a pneumatic tire comprising, on a tread face of the tire, a shoulder land portion extending in a circumferential direction of the tire, and a thin groove extending in the circumferential direction of the tire adjacent to a contact end of the shoulder land portion where comes into contact with the ground and segmenting the shoulder land portion into a main land portion located at the inner side thereof as viewed in the width direction of the tire and a sacrifice land portion located at the outer side thereof as viewed in the width direction of the tire.
2. Description of the Related Art
Conventionally, in order to prevent irregular wear of tires, it is widely took measures that a tire is formed, on a tread surface, with a thin groove that extends in the circumferential direction of the tire adjacent to a contact end of the shoulder land portion where comes into contact with the ground. However, when the tire receives a force from a road surface during running and when a sacrifice land portion thereof is deformed excessively, this causes stress convergence at the bottom of the groove and thereby results in cracks. Thus, conventional tires have had a problem in the aspect of the durability thereof.
Such cracks are easily generated at the groove bottom of a tire in a situation as described below. That is, for example, as shown in FIG. 5, such cracks are easily generated at the groove bottom of the tire particularly in the case where a sacrifice land portion 11 of the tire runs on the step 10 and is deformed excessively and the bottom of a thin groove 12 receives a force in a direction indicated with the arrowheads when a vehicle runs along a step 10 extending along a road shoulder in the vehicle's travel direction. In the situation mentioned hereinbefore, if the sacrifice land portion 11 of the tire is formed to be thinner, even when the sacrifice land portion 11 is deformed, the stress hardly affects on the groove bottom and cracks are hardly generated. However, in this case, there arises such a problem that the sacrifice land portion 11 itself is broken away or the preventive effect against the irregular wear of the tire is reduced.
As a measure to prevent a crack generation in the thin groove, for example, Japanese Published Patent Publication No. 2002-512575 discloses a technique such that, in order to increase the curvature radius at the groove bottom, the thin groove is formed, in the groove bottom, with an enlarged portion at the sacrifice land portion side to thereby prevent generation of cracks in the groove bottom.
However, for example, even when the curvature radius at the groove bottom is increased by forming an enlarged portion 16 in a thin groove 15 as shown in FIG. 3, in the case where the tire receives a lateral force and the thin groove 15 is deformed in a direction that the thin groove is squashed as shown in FIG. 4, the curvature radius at the groove bottom is not ensured. As a result, the crack generation cannot be satisfactorily prevented. In order to increase the preventive effect against the crack generation in the groove bottom, when the curvature radius of the enlarged portion 16 is further increased, a cavity of the enlarged portion 16 at the sacrifice land portion 14 side is inevitably enlarged. As a result, the rigidity of the sacrifice land portion 14 cannot be ensured. Therefore, there is a limitation for increasing the curvature radius at the groove bottom of the thin groove 15.